1. Field of the Invention
This invention relates to pressure-sensitive adhesives, and, more particularly, a composition formed from a blend of pressure-sensitive adhesives.
2. Discussion of the Art
Normally tacky pressure-sensitive adhesive compositions must have an appropriate balance of wetting ability or tack, adhesion, and cohesive strength. All pressure-sensitive adhesive compositions exhibit each of these properties in varying degrees.
Early pressure-sensitive adhesive compositions consisted of natural crude rubber tackified by a mixture of esterified wood rosin. Improved pressure-sensitive adhesive compositions were later realized upon the introduction of synthetic acrylic ester elastomers, which were inherently tacky and possessed improved cohesive and adhesive properties. In addition, they also exhibited low temperature holding power and high resistance to oxidation.
More recently, block copolymers, such as, for example, styrene-isoprene-styrene and styrene-butadiene-styrene, have been formulated into pressure-sensitive adhesive compositions having an excellent balance of tack, adhesion, and cohesive strength. They also exhibit hot melt processability and high values of peel adhesion to both polar and non-polar surfaces. However, they do not exhibit the resistance to oxidation or the high temperature holding power of the acrylic ester elastomer adhesives.
The development of styrene-ethylene/butylenestyrene block copolymers, which have a saturated olefin midblock, permit the formulation of adhesive compositions that exhibit high values of peel adhesion to both polar and non-polar surfaces and are resistant to oxidation and photochemical degradation. Like adhesive compositions that contain styrene-isoprene-styrene and styrene-butadiene-styrene block copolymers, they do not exhibit the high temperature holding power of adhesive compositions that contain acrylic esters. Moreover, they require higher levels of plasticizers or tackifying resins in order to achieve acceptable levels of quick wetting ability (tack). It has also been found that excessive dilution of the block copolymer with tackifiers compromises the cohesive strength of the adhesive.
It has frequently been proposed to blend two or more pressure-sensitive adhesive compositions in order to realize the advantages of each. However, mere blending in solution of two or more different pressure-sensitive adhesives most often results in a composition of poor, or at best, compromised properties, and consequently, the blend is of little or no commercial value as a pressure-sensitive adhesive. There are several reasons for these poor adhesive properties. First, a member from one class of polymers is generally not miscible with a member from another class of polymers. Second, a member from one class of polymers generally adheres poorly to a member from another class of polymers. Accordingly, the interfacial regions between the domains of each component are weak and result in cohesive failure when subjected to stress. Because of this failure, most pairs of polymers are said to be "thermodynamically incompatible". The two major classes of pressure-sensitive adhesives, acrylic adhesives and block copolymer adhesives, are thermodynamically incompatible. However, it would be desirable to combine acrylic adhesives and block copolymer adhesives in order to develop pressure-sensitive adhesive compositions that exhibit the preferred characteristics of each type of adhesive. For example, it would be desirable to develop an adhesive that exhibits the high tack, low temperature adhesion, and high cohesive strength of acrylic adhesives and the high peel adhesion values of block copolymer adhesives without compromising the hot melt processability and resistance to oxidation of both.